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04.02.116:52 AM ET

Why Planes Fall Apart

A Southwest flight from Phoenix to Sacramento was forced to make an emergency landing Friday when a hole opened in the roof. Aviation expert Clive Irving reports on why outsourcing may be a culprit—and the worst-case scenario.

Southwest Airlines grounded 81 of its oldest jets Saturday following an alarming emergency Friday night when the cabin of one of its Boeing 737s was ripped open at 36,000 feet.

In both cases there was a sudden loss of cabin pressure as part of the fuselage structure failed. In both cases the pilots had to get the airplane on the ground as fast as possible, making diversions.

And in both cases the airplane involved was the
Boeing 737—the only airplane Southwest flies, in a variety of models and ages.

Flight 2294 was en route from Nashville to Baltimore when passengers heard a loud pop and a hole the size of a football appeared in the roof. On Friday night, as Flight 812 was en route from Phoenix to Sacramento, there was a jarring blast and a hole opened up in the roof, far larger than a football.

As scary as these events were—and they are very scary—both airplanes landed safely and nobody was injured.

That is where the good news ends.

Such failures should never happen. Their cause is well known, and the prevention of them is a basic, well-rehearsed, and essential part of regular maintenance.

As an airliner climbs and reaches cruise altitude, there is an incremental contrast between the air pressure in the cabin, which is kept at the equivalent of about 8,000 feet, and the air outside. At cruise altitude, around 36,000 feet, the cabin is, in effect, like an inflated balloon. The skin of the airplane is the seal between the two pressures. If the skin fails, the balloon pops.

All airplanes are designed with more than enough integrity to prevent what is technically called explosive decompression. As airplanes age, their skins are prone to metal fatigue. Regular maintenance checks should ensure that fatigue cracks are detected long before they present a danger, and the skin panels are replaced.

Like many other U.S. carriers, Southwest outsources a lot of its maintenance.

That is why maintenance is so critical to safety.

Late last year, the Federal Aviation Administration proposed a fine of $530,250 on Southwest for lapses in maintenance, after finding three violations of safety procedures in two years. In 2009, the FAA found that one of Southwest’s maintenance contractors used unapproved parts on 82 airplanes, and that 44 of the airline’s 737s had made more than 100,000 flights “out of compliance.”

Like many other U.S. carriers, Southwest outsources a lot of its maintenance. And much of the work now done for our domestic airlines goes, after competitive bidding, out of the country, to places like Mexico, El Salvador, Singapore, and the Philippines.

It doesn’t follow that mechanics in those countries are any less competent than those in the U.S. But both in the U.S. and overseas, mechanics who are unlicensed and have not undergone screening are used. Stretched for staff, the FAA has relied more and more on analyzing statistics and reporting by the airlines themselves rather than on-site surveillance. This regime is far too lax.

The airline pioneered the business model of budget carriers with two fundamental innovations: using a single type of airplane, and turning those airplanes around between flights faster than any competitor.

As a result, Southwest set new standards for airplane utilization. The airline’s 737s, on average, make seven flights a day, being in the air for more than 12 hours.

The frequency of safety checks is based on what are called cycles of flight—in which the number of takeoffs and landings, rather than time in the air, are critical. The intensity of Southwest’s schedules means that the kind of stresses that can produce cracks in the fuselage skin should always be a number one priority of inspections.

The 737 involved in Friday night’s emergency was one of the oldest models in the fleet, a series 300 delivered in 1996. On Saturday, Southwest said that it was working with Boeing to inspect the 81 737s of this vintage—all of them subject to a specific FAA directive aimed at catching cases of skin fatigue in 300 series airplanes still flying. The inspections will take several days.

Inspectors from the National Transportation Safety Board will want to know when the 737 involved in Friday night’s emergency was last inspected for cracks—and where.

The Worst-Case Scenario:

The most dramatic example of a Boeing 737 suffering an explosive decompression occurred in April 1988, with
Aloha Airlines Flight 243, over Hawaii. An entire section of the roof ripped off, leaving passengers exposed as though sitting on the top deck of a roofless tour bus. One flight attendant was sucked out to her death and 65 passengers were injured but, miraculously, the 737 landed. It was found that the plane had well exceeded the number of flights it should have made for its age and that its skin had been corroded by the Hawaiian climate.

The worst-ever airline accident involving a single airplane was in Japan in August 1985 when a Japan Airlines 747 suffered an explosive decompression that robbed the pilots of directional control. The 747 crashed near Mount Fuji and all 520 people aboard died.